Final answer:
The diagram that plots the structure of a black hole as radius versus time is the Penrose diagram, which is distinct from the Hertzsprung-Russell diagram used for stars.
Step-by-step explanation:
The diagram that plots the structure of a black hole as radius versus time is known as the Penrose diagram. This diagram is different from the Hertzsprung-Russell diagram, which plots stellar luminosity against surface temperature for a group of stars. While the H-R diagram is widely used in astronomy to understand the characteristics and evolution of stars, the Penrose diagram is specific to the field of general relativity and represents the causal structure of spacetime regions, including those around black holes.